Conventionally, a hybrid system in which a secondary battery and a capacitor are combined is known as a power storage system that realizes both high energy density and high power density. However, the short life of the secondary battery is a hindrance to the installation of such a hybrid system in a carrier such as a ship. In order to overcome this problem, for example, there is a known hybrid power supply system that is proposed by Non-Patent Literature 1 as a long-lived hybrid system.
In the hybrid power supply system, a battery supplies a low-frequency component of electric power, and an electric double layer capacitor supplies a high-frequency component of electric power. The high-frequency component is separated by an HPF (High Pass Filter), and the time constant TEDLC of the HPF is set based on the capacity and charge/discharge cycle of the capacitor. In order to prevent overcharging and over-discharging of the capacitor, a proportional compensator is used. The proportional gain kp of the proportional compensator is set so that the battery can supply electric power to cover a loss in the capacitor.